1. Field of the Invention
The present invention relates to a piezo drive system for driving a body to be driven or displaced, using a piezo drive element and, more particularly, to a piezo drive system for rotationally driving a rotor.
2. Description of the Related Art
In the past, general techniques for driving lenses incorporated in optical devices (such as the taking lens of a camera, a projector lens included in an overhead projector, lenses in binoculars, and lenses in a copier) or in machines including a driven portion such as a plotter or X-Y driving table have used impact actuators as disclosed in the following patent references 1-8. In these techniques, a piezo element is slowly stretched and then quickly compressed. Alternatively, the element is quickly stretched and then slowly compressed. Thus, an inertial force and a frictional force are made to act alternately to produce a linear drive of a lens or lenses. Patent reference 1 sets forth a typical technique of the aforementioned actuator mechanism. The other patent references 2-8 describe how the actuator mechanism is used.    [Patent reference 1] JP-A-4-069070    [Patent reference 2] JP-A-11-18447    [Patent reference 3] JP-A-11-44899    [Patent reference 4] JP-A-11-75382    [Patent reference 5] JP-A-2000-19376    [Patent reference 6] JP-A-2003-141827    [Patent reference 7] JP-A-2003-317410    [Patent reference 8] JP-A-2004-56951
The mechanism of the above-described actuator mechanism is described by referring to FIGS. 24A-24C. FIG. 24A is a schematic perspective view of a piezo drive system. FIGS. 24B and 24C are graphs illustrating the relationship between the displacement magnitude of a piezo element and time. The actuator shown in FIG. 24A includes a piezo element 500, a shaft 502, a slider 504, and a lens 506. One face of the piezo element 500 is connected to the shaft 502, while the other face is secured to the body 508. The shaft 502 extends through the slider 504. The slider 504 is biased against the shaft 502 by a biasing means (not shown) to hold the shaft. The slider 504 is displaced along the shaft 502 via the frictional force acting between the slider 504 and the shaft 502 by the biasing means. When the slider 504 is displaced, the lens 506 mounted to the front end of the slider 504 is displaced in the direction of the arrow F24a or F24b. Motion of the front-end side of the shaft 502 is merely suppressed by a spring. The front-end side of the shaft 502 is not fixed.
If an electric signal that is asymmetric with respect to time as shown in FIG. 24B is applied to the piezo element 500 to drive it in such a way that the device is slowly stretched and then quickly compressed, when the piezo element 500 is slowly stretched, the shaft 502 moves in the direction of the arrow F24a. At this time, the slider 504 is frictionally moved together with the shaft 502. If the piezo element 500 is quickly compressed, the slider 504 is kept at rest by the inertia force. Only the shaft 502 is attracted in the direction of the arrow F24b. Consequently, the slider 504 moves relative to the shaft 502 in the direction of the arrow F24a. By repeating these operations, the slider 504 is linearly driven in the direction of the arrow F24a. 
On the other hand, if an electric signal that is asymmetric with respect to time as shown in FIG. 24C is applied to the piezo element 500 to drive it in such a way that it is quickly stretched and then slowly compressed, the slider 504 is linearly driven in the direction of the arrow F24b by the action reverse to the foregoing action. The system of driving the actuator as described so far is simple in structure. Therefore, it is put into practical use as an actuator for auto focus of a digital camera module for use in a cellular phone.
Patent reference 9 described below discloses a drive mechanism making use of four multilayered piezo elements. The technique of this patent reference 9 is described by referring to FIGS. 25A-25C. The drive mechanism has the four multilayered piezo elements 604A-604D on which a circular flat plate 600B of a stator 600 is fixedly mounted. The flat plate 600B has a pillar 600A in its center. The pillar 600A is fitted in an elastic body 603 of a substantially cylindrical rotor 602, the elastic body being made of hard rubber or the like. The multilayered piezo elements 604A-604D are fixed to a base 606. Drive circuits 605A-605D are connected with the multilayered piezo elements 604A-604D, respectively. The pillar 600A of the stator 600 is tilted and made to produce a rotation by applying alternating electric fields across adjacent ones of the multilayered piezo elements 604A-604D, the electric fields being 90° out of phase. The frictional force from the elastic body 603 rotates the rotor 602 along the rotation direction of the pillar 600A.
[Patent reference 9] JP-A-3-74180
Lens modules used in digital cameras for use in cellular phones have been required to achieve high functions such as higher pixel count optical elements, zooming, autofocusing, and anti-jitter stabilization at lower cost. The electromagnetic type needs magnetic coupling and so it is difficult to achieve further miniaturization. The electrostatic type needs higher voltage. In contrast with these types, the aforementioned piezoelectric provides higher energy density if the size is reduced. Therefore, the piezoelectric has attracted attention as a small-sized actuator.
However, in the background art shown in FIGS. 24A-24C, the slider 504 and shaft 502 are in contact with each other via a frictional force and, therefore, they tend to be firmly stuck together. Especially, the direction of displacement of the piezo element 500 is perpendicular to the direction in which the fixing strength acts. Displacement of the piezo element 500 does not directly act on cutting of the fixing strength or on suppression. Accordingly, in order to drive the slider 504, it is necessary to displace the piezo element 500 greatly. Hence, the slider is driven at low efficiency. It is difficult to drive the slider at low voltage.
In the known structure shown in FIGS. 25(A) and 25(B), the rotor 602 and stator 600 are in contact with each other at a location where the end angle of the pillar 600A is displaced maximally. That is, they are in contact with each other at one point. Under this condition, the rotor is driven. Therefore, it is difficult to obtain stable driving force. Furthermore, it is difficult to adjust the rotor 602 in pushing it against the stator 600. In addition, rotation may be suppressed because the bottom of the rotor 602 touches the circular flat plate 600B of the stator 600. Further, the symmetry of rotation vibrations is easily lost because the four multilayered piezo elements are arranged. It is difficult to control vibrations, and the structure is complex. Hence, a large number of steps are required to assemble the structure.